Avoid like the plague.
I used Greasemonkey before version 4, but ever since the webextensions were forced after Firefox 57, GM never got back to where it was previously. They tried to force new APIs without any backwards compatibility which was a terrible decision, and other decisions were made which were ultimately the downfall of this addon. And now it hasn't been updated for years...
It's just sad what happened to this once awesome addon.
Install another extension such as Tampermonkey instead, it's a billion times better than this.